1. Field of the Invention
The present invention relates to a power transmission device that transmits power to a driven portion, a lens barrel, and an image pickup apparatus.
2. Related Background Art
In a conventional image pickup apparatus such as a camera, the driving force of a motor is amplified through a speed reducing mechanism, and a lens barrel provided with a cam and a helicoid screw is rotated by the torque thus generated to thereby move the lens barrel. In this case, the movements of the lens barrel as a driven member can be obtained by counting the output pulse of a pulse encoder operatively coupled to the lens barrel, for example. At this time, the pulse encoder effects the pulse output by reading the number of revolutions of a pulse disk or pulse blade on the motor shaft by a detecting unit such as a photo interrupter.
In this connection, when a drive source of low rotational speed but high torque such as an ultrasonic motor is used, the reduction ratio of the speed reducing mechanism can be set smaller than in the case where a drive source of high rotational speed such as an electromagnetic motor is used. Also, high-precision resolution can be attained for the movements of the lens barrel. However, to attain requisite resolution with a lens barrel for which a small reduction ratio is set by using a low-speed motor, it is necessary, as disclosed in Japanese Patent Application Laid-Open No. H07-318780 and Japanese Patent Application Laid-Open No. H09-191675, for example, to additionally provide a multiplying unit so that the requisite number of pulses can be attained or to enlarge the encoder so that greater pulse output can be obtained per one rotation of the motor.
FIG. 4 schematically illustrates a zoom driving system of a conventional lens barrel in the case where a low-speed drive source is used. In the construction shown in FIG. 4, a multiplying gear train (multiplying means) is provided separately from a speed reducing gear train. The speed reducing gear train consists of a zoom motor 17, gears 18 to 21, and a drive ring 12. The multiplying gear train consists of a gear 13 and a gear 14, the gear 14 rotating with a pulse disk 16, which constitutes an encoder together with a photo interrupter 15, fixed thereto. Therefore, the output of the zoom motor 17 is multiplied, making it possible to obtain a greater number of pulses.
The above-proposed device, however, requires that the multiplying unit be additionally provided, lading to an increase in the size of the device. Likewise, the method of increasing the pulse output per one motor rotation by enlarging the encoder leads to an increase in the size of the device.
Further, with the device constructed as shown in FIG. 4, the gear having undergone speed multiplication in the multiplying gear train is not applied with a large load and thus rotates at a relatively high speed. Thus, the contact noise between the gear teeth increases, causing discomfort. As a conceivable solution to this problem, tension or load may be applied so as to prevent the gears from rolling uncontrollably, but this results in reduced output and involves a further increase in size.